I was wondering since google keeps making updates, I think its best if we are able to select format for our screenshots like JPG, PNG, latest thing that google recommends is WEBP which gives really good compression
So PNG are suiteable for line art and transparencies (neither of which apply to screenshots) JPEG is really the only choice.
When webp is universally supported by all browsers it would be ideal, but that day is not here yet. I'd rather not offer choice for every user but have 1 standard and at this time JPEG is still the most reliable and best choice for screenshots. Hopefully, that will change soon though as it would be nice to get them even smaller.